That's the problem with those skin-tight trousers... they're a real nuisance to get off.
A spiders body is covered in a hard outer skin known as an exoskeleton, which is really a skeleton, like ours, but on the outside of the body instead of the inside. As the spider's body grows the hard exoskeleton needs to be removed to allow for the expansion. You already knew that, right.
What you're seeing here is a spider climbing out of it's old exoskeleton and revealing the new one beneath. As you can see, wriggling out of multi-legged trousers, is no easy task. It's exhausting for the spider and leaves it vulnerable to predators.
At first the new skin is soft and pliable but after only a short while in contact with the air it becomes firm enough to support the spider's body.